1. Field of the Invention
The invention relates to an electrolyte membrane for a solid polymer fuel cell, and a production method for the electrolyte membrane, and also relates to a membrane-electrode assembly and a fuel cell that include the electrolyte membrane.
2. Description of the Related Art
Solid polymer type fuel cell batteries are known as a form of fuel cell batteries. The solid polymer type fuel cell battery, as compared with other forms of fuel cell batteries, is low in operation temperature (about 80° C. to 100° C.), and allows cost reduction and compact design, and is therefore regarded as a promising motive power source of a motor vehicle or the like.
In a solid polymer type fuel cell battery, as shown in FIG. 8, a membrane-electrode assembly (MEA) 50, a main component element, is sandwiched by separators 51, 51 that have a fuel (hydrogen) gas channel and an air gas channel, thus forming one fuel cell 52 that is called unit cell. The membrane-electrode assembly 50 has a structure in which an anode-side gas diffusion electrode 58a made up of a catalyst layer 56a and a gas diffusion layer 57a on the anode side is stacked on one side of a solid polymer electrolyte membrane 55 that is an ion exchange membrane, and a cathode-side gas diffusion electrode 58b made up of a catalyst layer 56b and a gas diffusion layer 57b on the cathode side is stacked on the other side of the solid polymer electrolyte membrane 55.
In the unit cell 52, it is necessary to secure a gas channel between the gas diffusion electrodes 58a, 58b and the separators 51 and prevent the leakage of gas to the outside of the cell and the mixture of the fuel gas and the oxidant gas. The stacking of the gas diffusion electrodes 58a, 58b on the surfaces of the electrolyte membrane 55 forms a stepped surface in the membrane-electrode assembly 50. Therefore, when the membrane-electrode assembly 50 is sandwiched by separators 51 to form a fuel cell 52, a gap that is formed due to the step needs to be sealed. In ordinary cases, therefore, a sealing-purpose resin material 59 is applied onto the surfaces of the electrolyte membrane 55 extending out from an end side of the gas diffusion electrodes 58a, 58b, to such a height that the rein material 59 reaches the separators 51. Then, the resin material 59 is hardened by heating to form a seal portion, thereby securing a sealing characteristic.
Another technology of sealing the gap caused by the aforementioned step in the fuel cell 52 is disclosed in the Japanese Patent Application Publication No. JP-A-2006-4677. That is, as shown in FIG. 9, a seal member 59a made of a rubber-like elastomer or the like which has a protrusion 59b is provided so as to cover a portion 55a of the electrolyte membrane 55 which extends sideway from the end portions of the gas diffusion electrodes 58a, 58b of the electrolyte membrane 55. When the membrane-electrode assembly 50 is sandwiched by separators 51 to form a fuel cell 52, the protrusion 59b enters, in a pressed contact fashion, a recess portion 51a formed in a separator 51.
Incidentally, the electrolyte membrane used in the solid polymer fuel cell is mainly a thin film of perfluorosulfonic acid polymer (Nafion, by DuPont, USA), that is, an electrolyte resin (ion exchange resin). Besides, a thin film made of an electrolyte resin alone does not achieve sufficient strength. Therefore, a technology described in Japanese Patent Application Publication No. JP-A-9-194609 uses a reinforced type electrolyte membrane formed by impregnating a porous reinforcement film (e.g., a thin film formed by stretching PTFE, a polyolefin resin, etc.) with a solvent-dissolved polymer (electrolyte resin), and then introducing ion exchange groups in the electrolyte polymer after it is dried.
As described above, in order to seal the gaps between the electrolyte membranes and the separators which are formed as a result of the steps formed on membrane-electrode assemblies, the fuel cells of the related-art technology adopt either the application of a seal material or member, that is, a separate material, onto the edge portion of the electrolyte membrane, or the covering of the edge portion of the electrolyte membrane with a seal member that is made of a separate material. In the method in which a seal material is applied and is hardened by heating, non-uniform application of the seal material is likely to allow leakage, or the requirement of long-time heating or hardening by heating results in damages to the membrane-electrode assembly. Furthermore, non-uniform heat-hardening of the seal material also leads to leakage.
In the case of seal means in which a seal member made of a rubber-like elastomer or the like is provided to cover the edge portion of the electrolyte membrane, the production thereof is not easy, although high sealing effect can be expected due to the pressed contact of the seal member to the separator side. Furthermore, there is an inconvenience of the seal member being liable to positional deviation.
Furthermore, in either one of the related-art sealing technology in the fuel cells, a material different from the electrolyte membrane is interposed between the electrolyte membrane and the separators, and therefore a boundary surface is inevitably formed between the electrolyte membrane and the seal member, and there is a risk of the boundary surface causing breakage of seal.